An emulsion includes a liquid phase substance immiscible with a continuous liquid phase, which is dispersed in the continuous liquid phase. Emulsions, such as an O/W emulsion, in which oil droplets are dispersed in a continuous aqueous phase, and a W/O emulsion, in which aqueous droplets are dispersed in a continuous oil phase, are generally known. Further, it is known that such emulsions can be produced by an interface chemical method using an emulsifier or by a mechanical method using a specific emulsification apparatus. These two methods are generally used in combination to produce a stable emulsion. However, in the latter mechanical method, it is generally known that properties (e.g., droplet diameter of the dispersion phase and droplet diameter distribution thereof) of a resulting emulsion are largely varied depending on the emulsification apparatus used.
Currently, emulsions occupy important positions as raw materials and products in various industrial fields, for example, in the fields of cosmetics, food, paint, paper manufacture, film, recording material and the like. As the properties of such emulsions, the particle size and particle size distribution of the droplets that form the dispersion phase are important factors which seriously affect the stability of the emulsion or properties of a final product. Particularly, in a cosmetic emulsion or the like, the compatibility to the skin varies depending on the average particle size and particle size distribution of the emulsified and dispersed droplets. Further, the product stability thereof is also seriously affected thereby.
A microcapsule having a polymeric membrane or the like formed at an interface between the continuous phase and the dispersion phase of an emulsion, or a polymer fine particle obtained by polymerizing an emulsion liquid comprised of a polymeric dispersion phase is produced by treating the emulsion through processes such as polymerization, filtration and washing, drying, sieving, and breaking up of aggregate. Such microcapsules or polymer fine particles are also used in various industrial fields. The microcapsules are used as information recording material using pressure sensitivity, heat sensitivity and photosensitivity as their characteristics, including toner for copying machines and printers, as display material such as electronic paper, and further as medicine, pesticide, insecticide, fragrance, thermal storage medium and the like. The polymer fine particles are used as an antiblocking agent for plastic film, as an optical material for providing light diffusion/reflection preventing functions or for spacer use, as paint and ink for providing functions such as frosting, coloring and tactile sensation to building materials or automotive interiors, as cosmetic material for providing a slipping property to foundation or the like, as resin additive for improving heat resistance, solvent resistance or low shrinkage property, and further as a diagnostic testing agent and particulate formulation in medical field. The microcapsules and polymer fine particles are used, in addition, for various purposes such as pigment, dyestuff, conductive member, thermosensitive recording paper, resin reinforcement, grease additive, artificial stone, chromatography and the like. Since the particle size and particle size distribution of generated particles are substantially determined in these microcapsules and polymer fine particles during the stage of emulsification, it is not an exaggeration to say that the properties of an emulsion determine the final performances of a product. Therefore, development of an emulsification apparatus, capable of easily producing a product having desired average particle size and particle distribution, particularly, a narrow particle size distribution, is needed regardless of whether or not the product is used in a form of emulsion or in a form of microcapsule or polymer fine particle.
Various methods are proposed for the mechanical production of emulsions. The most common emulsification method comprises feeding raw materials into a batch tank and agitating the contents in the tank by a shearing blade rotating at high speed. However, this method can be problematic due to the formation of non-uniform particle size of the discontinuous phase (i.e., the dispersion phase) in a final emulsion or residue of unemulsified raw materials due to the tendency of non-flowing parts to remain within the tank, or difficulty in scale-up. An apparatus having an agitating device that is separate and distinct from the shearing blade, can be adapted to cause the entire contents in the tank flow can be a countermeasure to prevent such problems, is also proposed, it is extremely difficult to perfectly solve the problems. Further, an increased cost is needed for scale-up since the shearing blade and a drive unit thereof must be enlarged therefor. This method is disadvantageous also from the point of maintenance since the drive part, which rotates at high speed, has a precision structure. Further, when the emulsifying amount is large, denaturation of the contents may be caused during the emulsifying operation since the emulsifying operation takes a long time.
In order to solve the above-mentioned problems, a method for continuously performing emulsification is also proposed.
Japanese Patent Application Laid-Open No. H5 (1993)-49912 for example, discloses continuous emulsification that is carried out by rotating an agitating blade having a specific tip shape at high speed in a narrow area within a pipe and introducing raw materials into the narrow area between an outer wall and the tip of the agitating blade. In this method, since the shearing force is determined based on the rotation of the blade, an extremely large power output part is needed when a large shearing force is required, or when an emulsion having small dispersion phase droplets is to be obtained. In addition, a problem occurs such that when the emulsifying amount is increased, an emulsion liquid having a dispersion phase with a uniform particle size distribution cannot be obtained since the residence time in the emulsification apparatus is shortened. Further, the agitating blade is difficult to fabricate and maintain due to the complicated shape of its tip and a very narrow clearance between the tip and the outer wall.
Japanese Patent Application Laid-Open No. H6 (1994)-142492 discloses an emulsification apparatus that includes a preliminary mixing tank of raw materials is needed, and wherein emulsification is performed by passing the raw material mixture through a subsequent emulsion machine (in line) in which the shearing force is continuously changed. According to this method, an emulsion having a wide particle size distribution can be obtained, the emulsion being characteristically free from extremely large particles or extremely small particles. In this method, however, since the raw material loading amount and the number of rotations of the emulsifying machine must be controlled, the operation becomes complicated. Further, if a material to be emulsified is reactive, clogging may result.
Japanese Patent Application Laid-Open No. H9 (1997)-029091 discloses that emulsification is carried out by continuously feeding raw materials from the bottom of a kiln, agitating the content in the kiln, and continuously extracting from an upper portion of the kiln an amount of the content which is equivalent to the amount to be loaded. With this method, clogging is never caused within the emulsification apparatus even if the raw material to be emulsified is a reactive compound. However, when the emulsification rate is raised, deterioration of the particle size distribution of the dispersion phase and short-pass discharge of unemulsified raw materials can result in the worst case.
Japanese Patent Application Laid-Open No. H5 (1993)-212270 discloses a continuous emulsification method using a porous glass pipe. In this method, an expensive apparatus is needed, and clogging of the porous glass pipe may result if the raw material is reactive. The particle size of the emulsion is determined by the pressure at the time of pushing raw materials to be emulsified out of the porous glass pipe and the flowing state of a fluid which can form a continuous phase. Therefore, controlling the particle size becomes complicated and difficult. Further, since the porous glass pipe is expensive, a problem may be caused such that an increased cost is needed for scale-up.
Further, Japanese Patent Application Laid-Open No. H2 (1990)-261525 and Japanese Patent Application Laid-Open No. H9 (1997)-201521 disclose methods for instantaneous emulsification by making raw materials to be emulsified collide with each other at super-high pressure and high speed. Such an apparatus requires an apparatus body having a robust structure due to serious wear that results from extremely high operating pressure. Further, the emulsifying effect is difficult to control since its emulsification is based on the impact force of the collision of the raw materials to be emulsified. As a result, particle size distribution of dispersion phase droplets in the emulsion liquid becomes remarkably nonuniform.
Japanese Patent Application Laid-Open No. 2000-254469 and Japanese Patent Application Laid-Open No. 2002-28463 disclose emulsification apparatuses having a structure in which two or more sheet-like elements divided into a number of polygons by barrier walls or sheet-like elements having a number of pore parts are directly superposed. With these apparatuses, mixing or emulsification of raw materials to be emulsified is carried out by passing the raw materials through divided flow passages formed by the two or more sheet-like elements. However, this method requires a strict adjustment for layout of each element within the apparatus, in addition to the complicated shape of the elements used. Further, with emulsification apparatuses utilizing the division method, the division effect is reduced when the particle size of dispersion phase droplets in the emulsion liquid becomes smaller, and the emulsifying effect of the apparatus itself is consequently reduced.
Finally, Japanese Patent Application Laid-Open No. 2002-159832 discloses an emulsification apparatus having a structure composed of two or more spaces partitioned by barrier walls having one or more small pores. The disclosed apparatus is adapted to emulsify the raw materials to be emulsified by pulverizing and fragmenting the raw materials using a strong impact force when introducing the raw materials into an adjacent space at high speed and high pressure through the small pores. However, the particle size distribution of the emulsion liquid obtainable in principle tends to be nonuniform since the fragmentation phenomenon by impact is difficult to control. Namely, only the fragmentation phenomenon by impact is used as the principle of emulsification. Further, the emulsification apparatus needs a robust structure for introducing the raw materials under high pressure.
As described above, the conventionally proposed continuous emulsification methods and apparatuses had problems such as poor uniformity of dispersion phase droplets in a resulting emulsion liquid, difficulty in scale-up, complexity of apparatus and complication of maintenance, thus were not sufficiently satisfactory.